In recent years, in view of environmental protection, capacitors used for assisting power at the time of activating a vehicle, a construction machine, and the like have been developed. For capacitors for this purpose, there have been demands for increasing an electric capacity per unit volume and for reducing an internal resistance. That is, a large energy density and a high power density have been demanded.
In order to reduce the internal resistance of a capacitor so as to satisfy these demands, it is essential to provide a structure that has high adhesion between a current collector and a polarizing electrode material. As the polarizing electrode material, carbon-based powder particles having high conductivity, such as activated carbon, are generally used.
For example, Patent Literature 1 discloses an electrode for capacitor in which the adhesion between the current collector and the polarizing electrode material is enhanced. FIG. 8 is a cross-sectional view showing an electrode for capacitor in Patent Literature 1.
The electrode includes current collector 101, polarizing electrode layer 102 and anchor coat layer 103 formed on current collector 101. Anchor coat layer 103 is formed of conductive particles (for example, carbon black) and a binder, and is provided between current collector 101 and polarizing electrode layer 102. According to this structure, particles of carbon black in anchor coat layer 103 provided between current collector 101 and polarizing electrode layer 102 are thrust into fine voids of polarizing electrode layer 102. Due to an anchor effect obtained by this structure, a joining strength of polarizing electrode layer 102 to current collector 101 is improved.
Moreover, anchor coat layer 103 is provided so as to embed gaps that exist between current collector 101 and polarizing electrode layer 102. For this reason, the volume of the gaps located between current collector 101 and polarizing electrode layer 102 can be made smaller. As a result, the adhesion of polarizing electrode layer 102 to current collector 101 is improved. Thus, it becomes possible to reduce the internal resistance of the capacitor.
However, even if the electrode for capacitor having the above structure is used, the internal resistance is not sufficiently reduced. In particular, in order to quickly charge/discharge a large quantity of electricity as a capacitor for assisting power of a vehicle or a construction device, the resistance between current collector 101 and polarizing electrode layer 102 needs to be further reduced.
Moreover, upon repeating charging/discharging processes as a capacitor, pH of the electrolytic solution is changed, with the result that acid or alkali accumulated in the electrolytic solution during this time comes to react with current collector 101 to cause corrosion therein. This corrosion is one factor of an increase in the internal resistance of the electrode for capacitor.